There are many reasons that it is of interest to industry to heat materials. One method of heating material is to flow the material through a rotating drum and apply heat to the outside of the drum or a flame on the inside of the drum. A flame on the inside of the drum is sometimes undesirable because of the products of combustion produced by the flame, and sometimes exposing the material directly to the flame is undesirable. A flame on the outside of the drum is sometimes undesirable because the drum wall must be made of material that will withstand high temperature. In fact, for very high temperatures, there is no suitable material that is a good heat conductor and will withstand a flame directly on the outside of the drum wall.
It is a well known fact that electromagnetic waves can impart heat to a material when the material absorbs the energy from such waves. Infrared radiation, that portion of the electromagnetic spectrum that extends from the long wavelength, or red, end of the visible light range to the microwave range, is absorbed by window glass and water, but quartz is transparent to infrared radiation. However, window glass is transparent to visible light frequencies. Microwave energy is reflected by some materials, such as aluminum and stainless steel, absorbed by other materials, such as water and tungsten carbide, while other materials such as quartz and teflon are transparent to microwaves. For example, a potato in a microwave oven can be heated by the microwave energy while the ceramic saucer holding the potato cannot. The ceramic saucer holding the potato will get hot by conduction of heat from the potato but will not get as hot as the potato, since the potato is essentially being heated from the inside out.
Accordingly, a good clean method for heating certain materials is to use microwaves; however, most methods that use microwaves today are "batch" processes. Industry needs an improved "continuous" process for applying electromagnetic waves to certain materials.
The use of rotating drums to convey materials while using heat to modify such material, either chemically or physically, is well known in the art. Examples of such an apparatus may be found, for example, in my U.S. Pat. No. 4,872,954, issued Oct. 10, 1989; U.S. Pat. No. 5,078,836, issued Jan. 2, 1992; U.S. Pat. No. 5,227,026, issued Jul. 13, 1993; U.S. Pat. No. 5,523,060, issued Jun. 4, 1996; and application Ser. No. 08/755,671 filed Nov. 25, 1996.